Two beam interferometers split a light beam into two light beams, which are typically directed over different paths and then recombined so that the light beams interfere with each other. The interfering beams produce an interference pattern which may be projected onto a surface. A phase stepping technique may be used to vary the phase difference between the interfering light beams, facilitating measurement of characteristics of the surface based on the interference pattern at the various phase steps. Varying elevations over a region of a surface, small movements of the surface and the characteristics of optical devices such as a lens can be analyzed by such a technique.
FIG. 1 illustrates a prior art Michelson interferometer 10 that is often used to provide the phase stepping needed for typical applications. A beam of spatially and temporally coherent light 12 is projected on a beam splitter 14, which divides the light into two beams 16 and 18. The light beams 16, 18 are projected onto mirrors 20 and 22, respectively. The mirrors 20 and 22 reflect the light beams 16 and 18, respectively, back to the beam splitter 14, which recombines the beams and directs the recombined beam toward the fringe observation plane 24. The fringe spacing "d" in the observation plane 24 is determined by d=.lambda./2sin .phi., where .phi. is the angle between the beams 16, 18 and .lambda. is the light wavelength. As the difference in the phase between the two interfering beams 16, 18 is varied, the fringes move across the field of view.
The relative phase between the light beams is changed by a mechanical device which translates one of the mirrors, such as mirror 22, along the direction of light propagation in small steps (a fraction of a wavelength). At least three images are obtained, wherein the maxima of the projected interference fringe in each image occurs at a different lateral position. The steps in lateral fringe position correspond with the steps in the relative phase between the two interfering beams. The visibility of the fringes as the mirror is displaced depends upon the degree of longitudinal coherence of the source. Because the sensitivity of such an interferometer needs to be very high, the element controlling the phase shift must have high mechanical precision. The successful construction of such a device, therefore, demands great skill.
Because the two beams 16, 18 traverse different paths, they may be subject to different environmental effects, such as vibration, temperature differences and noise, which can cause deviations in the resulting interference pattern. Such deviations can introduce significant errors into the analysis of surface heights and the motion of precision tooling, microscopes and lithographic equipment, for example, as well as in the analysis of optical systems.
In addition, the mapping of surface elevation by profilometers of the prior art, including interferometric varieties, is often a slow process. While mapping of the surface of skin is useful for dermatological, cosmetic and pharmaceutical applications, a living subject cannot be restrained from movement long enough to successfully map the surface height over a meaningful area. Optically opaque silicone rubber casts are therefore taken of the skin surfaces and the casts are separately analyzed for surface height. When taken of curved surfaces such as skin, however, such casts often do not maintain the shape of the curved surface and do not faithfully replicate the height variations of skin.
U.S. Pat. No. 5,434,669 to Tabata et al. ("Tabata") shows an interferometric device for use in endoscopy, including embodiments where electrooptic and liquid crystals are used to cause a phase shift between polarized components of a light beam. (See FIGS. 21, 22). While providing some improvement over mechanical phase shifting devices, the light is conveyed between the optical components of the system and onto the area of interest through optical fibers, which can alter the phase retardation produced by the electrooptic or liquid crystals in an uncontrollable manner, due to thermally and mechanically induced instabilities in the transmission of the light. In addition, the optical fibers are said to be "polarization-maintaining". While such fibers have been found to maintain the state of the polarization of the light, i.e., linear polarization, they do not necessarily maintain the direction of polarization. The use of optical fibers limits the size of the surface which can be accurately analyzed, as well. Tabata's devices are therefore inappropriate for topographic imaging of skin over large fields of view at high resolution, in times sufficiently short to prevent blur due to motion of the subject.